1. Field of the Invention
The present invention relates to a digital camera for wirelessly transmitting photo-taken image data to an external device, and a controlling method therefor.
2. Description of the Related Art
Conventionally, when an image captured with a digital camera is transmitted to an external device using a wireless transmission system such as a wireless local area network (LAN), image data has been transferred by a method as illustrated in FIG. 7.
In FIG. 7, an image signal obtained by shooting a subject with an image sensor 100, such as a CCD, is temporarily stored in an internal memory 104 via an image processing unit 101.
Then, the image processing unit 101 reads out the image signal stored in the internal memory 104 to carry out a predetermined image process (matrix process→γ conversion→JPEG conversion). The image data subjected to the image process is recorded from the internal memory 104 to an external storage medium such as a memory card 105 attached to a digital camera (or they can be recorded in a storage medium housed inside the digital camera).
When the image data subjected to the image process is transferred to an external device using a wireless transmission system such as a wireless LAN, the image processing unit 101 reads out the image data obtained by the image process which is stored in the internal memory 104, or the image data stored in the memory card 105. Then, it transfers the data to a network processing unit 102. At the network processing unit 102, the protocol conversion, such as the conversion to the transmission control protocol/internet protocol (TCP/IP), is carried out to transfer the image data to the external device.
The image processing unit 101 and the network processing unit 102 can be formed integrally as a data processing system B. The network processing unit 102 or the data processing system B including the image processing unit 101 and the network processing unit 102 is connected with an oscillation circuit 103 configured to determine the operation frequency thereof. The oscillation circuit 103 configured to supply a clock signal to the network processing unit 102 or the data processing system B determines the optimum operation frequency to achieve the performance of the network processing unit B and the image processing unit.
The network processing unit 102 converts the data into protocol data according to the transfer method (i.e., packet) and transfers the data to a wireless communication unit 106 to carry out the radio frequency (RF) conversion for the wireless transmission at the wireless communication unit 106.
In the wireless communication unit 106, first the baseband process is carried out by the primary modulation operation, and then, the RF process for performing conversion to the frequency band of the wireless transmission (for example, in the case of the IEEE 802.11b/g, 2.4 GHz, and in the case of the IEEE 802.11a, 5 GHz)) is executed. Thereafter, it is converted to the final analog data via an output circuit (not shown), such as a power amplifier, to output an electric wave having a predetermined frequency band range to the outside via an antenna 107.
On the data receiving side, the electric wave transmitted from the wireless communication unit 106 of the digital camera is received at a receiving antenna 108.
In a wireless communication unit 109, first, the signal is received by a receiving amplifier to carry out RF conversion, which is inverse of the one performed at the time of the transmission. Next, the signal is converted to a packet data by the inverse modulation to the one performed at the time of the transmission. The converted packet data is transferred from the wireless communication unit 109 to an external device 110 serving as the host appliance, such as a personal computer (PC). The transferred data is stored as image data in a memory unit provided inside or outside the external device 110. The wireless communication unit 109 can be housed in the external device 110.
The data received by the wireless communication unit 109 is demodulated to the original image data, which is displayed on the monitor screen of the external device 110 in the order the images were taken, such as the first frame image, the second frame image, etc. For example, thumbnail images of the captured images are displayed on divided areas of the screen as illustrated on a monitor display image 111.
The wireless communication unit 106 includes a module for both transmission and reception. The module includes a processing unit configured to convert the image data to be transmitted into wireless RF data, and a data processing unit configured to receive wireless data from an external device which converts the received data into predetermined format data.
In the case of such a system in which an image is transmitted between a digital camera and an external device using the conventional wireless communication module, a mechanism is incorporated for reciprocally monitoring a state of the electric wave or the packet at any time on the transmitting and receiving sides. For example, in the case where a state of the electric wave received on the receiving side is poor, or errors of the received data are large in number (the data includes also data for error detection), the transmission and reception can be performed smoothly by switching the primary modulation method.
In general, as the method for the primary modulation, a digital modulation system such as phase shift keying (PSK) (QPSK, 8PSK) and quadrature amplitude modulation (QAM) (16QAM, 64QAM) is used. In such a modulation system, the transmission efficiency is improved by using the multi-value process. Therefore, the larger transmission data amount per one wavelength, the higher transmission speed.
According to the transmission system such as the wireless LAN, the communication speed can be changed by switching the modulation method. However, the processing of an image processing unit or a network processing unit of a digital camera is done at a constant operation frequency regardless of the operation state of the wireless communication unit. In such a case, a mechanism has not been considered that switches the control depending on information received from the wireless controlling unit that changes at any time.
Therefore, even in the case where the communication speed of the wireless communication unit is changed according to the state of the electric wave of the wireless communication, the processing speed (process clock frequency) of the processor that executes the network process has always been constant. Since the processor is always operated in a state that achieves the maximum performance, that is, at the highest frequency, the system as a whole unnecessarily consumes the energy.
For devices that are battery driven, such as a digital camera, such an operation is degrades performance of the device. For example, in the case of a digital camera, the number of photographs that can be taken and the number of wireless transmissions that can be performed are affected.
Japanese Patent Application Laid-Open No. 2002-320136 discusses a technique in which the driving frequency of the central processing unit (CPU) in a digital camera connected with an external appliance via a universal serial bus (USB) is changed according to the speed of the USB communication. However, in general, in the wired connection with a USB or an IEEE 1394, unless the connection state of the connected appliance (the order of the connection and the connection form which are named as topology) is changed, the communication speed stays constant. Therefore, after the connection state is initially detected, the communication speed is constant so that the driving frequency of the CPU of the digital camera is maintained constant as well.
On the other hand, in the case of a digital camera which is wirelessly connected, the state of electric wave changes at any given time. Therefore, when a process is operated according to the communication speed at the time of the connection, if the situation of the electric wave changes after establishing the connection, an optimum operation cannot be performed. For example, if an operation is started in maximum performance set immediately after establishing the connection and then the communication speed is lowered, energy is unnecessarily consumed in the whole system. On the other hand, if the communication speed is increased after starting the connection, the process cannot keep up with the increased speed and cannot obtain sufficient performance at the communication speed set at the time of starting the connection.